Peptide Hormones

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Structure and Function of Hormones

The integration of body functions in humans and other higher organisms is carried out by the nervous system, the immune system, and the endocrine system. The endocrine system is composed of a number of tissues that secrete their products, endocrine hormones, into the circulatory system; from there they are disseminated throughout the body, regulating the function of distant tissues and maintaining homeostasis. In a separate but related system, exocrine tissues secrete their products into ducts and then to the outside of the body or to the intestinal tract. Classically, endocrine hormones are considered to be derived from amino acids, peptides, or sterols and to act at sites distant from their tissue of origin. However, the latter definition has begun to blur as it is found that some secreted substances act at a distance (classical endocrines), close to the cells that secrete them (paracrines), or directly on the cell that secreted them (autocrines). Insulin-like growth factor-I (IGF-I), which behaves as an endocrine, paracrine, and autocrine, provides a prime example of this difficulty.

Hormones are normally present in the plasma and interstitial tissue at concentrations in the range of 10^-7M to 10^-10M. Because of these very low physiological concentrations, sensitive protein receptors have evolved in target tissues to sense the presence of very weak signals. In addition, systemic feedback mechanisms have evolved to regulate the production of endocrine hormones.

Once a hormone is secreted by an endocrine tissue, it generally binds to a specific plasma protein carrier, with the complex being disseminated to distant tissues. Plasma carrier proteins exist for all classes of endocrine hormones. Carrier proteins for peptide hormones prevent hormone destruction by plasma proteases. Carriers for steroid and thyroid hormones allow these very hydrophobic substances to be present in the plasma at concentrations several hundred-fold greater than their solubility in water would permit. Carriers for small, hydrophilic amino acid-derived hormones prevent their filtration through the renal glomerulus, greatly prolonging their circulating half-life.

Tissues capable of responding to endocrines have 2 properties in common: they posses a receptor having very high affinity for hormone, and the receptor is coupled to a process that regulates metabolism of the target cells. Receptors for most amino acid-derived hormones and all peptide hormones are located on the plasma membrane. Activation of these receptors by hormones (the first messenger) leads to the intracellular production of a second messenger, such as cAMP, which is responsible for initiating the intracellular biological response. Steroid and thyroid hormones are hydrophobic and diffuse from their binding proteins in the plasma, across the plasma membrane to intracellularly localized receptors. The resultant complex of steroid and receptor bind to response elements of nuclear DNA, regulating the production of mRNA for specific proteins.

Receptors for Peptide Hormones

With the exception of the thyroid hormone receptor, the receptors for amino acid-derived and peptide hormones are located in the plasma membrane. Receptor structure is varied: some receptors consist of a single polypeptide chain with a domain on either side of the membrane, connected by a membrane-spanning domain. Some receptors are comprised of a single polypeptide chain that is passed back and forth in serpentine fashion across the membrane, giving multiple intracellular, transmembrane, and extracellular domains. Other receptors are composed of multiple polypeptides. For example, the insulin receptor is a disulfide-linked tetramer with the β-subunits spanning the membrane and the α-subunits located on the exterior surface.

Subsequent to hormone binding, a signal is transduced to the interior of the cell, where second messengers and phosphorylated proteins generate appropriate metabolic responses. The main second messengers are cAMP, Ca²⁺, inositol triphosphate (IP₃), and diacylglycerol (DAG). The generation of cAMP occurs via activation of G-protein coupled receptors (GPCRs) whose associated G-proteins activated adenylate cyclase. For more information on GPCRs and G-proteins visit the Signal Transduction page. Adenylate cyclase then converts ATP to cAMP and the subsequent increases in cAMP lead to activation of cAMP-dependent protein kinase (PKA) as shown in the Figure below. GPCRs also couple to G-protein activation of phospholipase C-γ (PLCγ). Activated PLCγ hydrolyzes membrane phospholipids (as described below) resulting in increased levels of IP₃ and DAG. Downstream signaling proteins are phosphorylated on serine and threonine by PKA and DAG-activated protein kinase C (PKC) leading to alterations in their activities. Additionally, a series of membrane-associated and intracellular tyrosine kinases phosphorylate specific tyrosine residues on target enzymes and other regulatory proteins.

The hormone-binding signal of most, but not all, plasma membrane receptors is transduced to the interior of cells by the binding of receptor-ligand complexes to a series of membrane-localized GDP/GTP binding proteins known as G-proteins. The classic interactions between receptors, G-protein transducer, and membrane-localized adenylate cyclase are illustrated below using the pancreatic hormone glucagon as an example. When G-proteins bind to receptors, GTP exchanges with GDP bound to the α subunit of the G-protein. The G_α-GTP complex binds adenylate cyclase, activating the enzyme. The activation of adenylate cyclase leads to cAMP production in the cytosol and to the activation of PKA, followed by regulatory phosphorylation of numerous enzymes. Stimulatory G-proteins are designated G_s, inhibitory G-proteins are designated G_i. For more information on G-proteins and GPCRs visit the Signal Transduction page.

Representative pathway for the activation of cAMP-dependent protein kinase, PKA. In this example glucagon binds to its' cell-surface receptor, thereby activating the receptor. Activation of the receptor is coupled to the activation of a receptor-coupled G-protein (GTP-binding and hydrolyzing protein) composed of 3 subunits. Upon activation the α-subunit dissociates and binds to and activates adenylate cyclase. Adenylate cylcase then converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. Once released the catalytic subunits of PKA phosphorylate numerous substrate using ATP as the phosphate donor.

A second class of peptide hormones induces the transduction of two second messengers, DAG and IP₃ (diagrammed below for α-adrenergic stimulation by epinephrine). Hormone binding is followed by interaction with a stimulatory G-protein which is followed in turn by G-protein activation of membrane-localized PLCγ. G-proteins that are coupled to receptor-activation of PLCγ are termed G_q-proteins. PLCγ hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce two messengers: IP₃, which is soluble in the cytosol, and DAG, which remains in the membrane phase. Cytosolic IP₃ binds to sites on the endoplasmic reticulum, opening Ca²⁺ channels and allowing stored Ca²⁺ to flood the cytosol. There it activates numerous enzymes, many by activating their calmodulin or calmodulin-like subunits. DAG has two roles: it binds and activates protein kinase C (PKC), and it opens Ca²⁺ channels in the plasma membrane, reinforcing the effect of IP₃. Like PKA, PKC phosphorylates serine and threonine residues of many proteins, thus modulating their catalytic activity.

Pathways involved in the regulation of glycogen phosphorylase by epinephrine activation of α-adrenergic receptors (see the Glycogen page for details of the regulatory mechanisms). PLCγ is phospholipase C-γ.

Only one receptor class, that for the natriuretic peptides (e.g. atrial natriuretic peptide, ANP: also sometimes called atrial natriuretic factor, ANF), has been shown to be coupled to the production of intracellular cGMP. ANP, a peptide secreted by cardiac atrial tissue, is much like other peptide hormones in that it is secreted into the circulatory system and has effects on distant tissue. The principal site of ANP action is the kidney glomerulus, where it modulates the rate of filtration, increasing Na⁺ excretion in the urine. The receptors for the natriuretic factors are integral plasma membrane proteins, whose intracellular domains catalyze the formation of cGMP following natriuretic factor-binding. Intracellular cGMP activates a protein kinase G (PKG), which phosphorylates and modulates enzyme activity, leading to the biological effects of the natriuretic factors.

Basics of Peptide Hormones

Many amino acid and peptide hormones are elaborated by neural tissue, with ultimate impact on the entire system. When their composition was still unknown, hypothalamic secretory products were known as releasing factors, since their effect was to release endocrine hormones from the pituitary. More recently the releasing factors have been renamed releasing hormones. Currently, both names are in common use.

Releasing hormones are synthesized in neural cell bodies of the hypothalamus and secreted at the axon terminals into the portal hypophyseal circulation, which directly bathes the anterior pituitary. These peptides initiate a cascade of biochemical reactions that culminate in hormone-regulated, whole-body biological end points. Cells of the anterior pituitary, with specific receptors for individual releasing hormones, generally respond through a Ca²⁺, IP₃, PKC-linked pathway that stimulates exocytosis of preexisting vesicles containing the various anterior pituitary hormones. The pituitary hormones are carried via the systemic circulation to target tissues throughout the body. At the target tissues they generate unique biological activities.

The secretion of hypothalamic, pituitary, and target tissue hormones is under tight regulatory control by a series of feedback and feed- forward loops. This complexity can be demonstrated using the growth hormone (GH) regulatory system as an example. The stimulatory substance growth hormone releasing hormone (GRH) and the inhibitory substance somatostatin (SS) both products of the hypothalamus, control pituitary GH secretion. Somatostain is also called growth hormone-inhibiting hormone (GIH). Under the influence of GRH, growth hormone is released into the systemic circulation, causing the target tissue to secrete insulin-like growth factor-1, IGF-1. Growth hormone also has other more direct metabolic effects; it is both hyperglycemic and lipolytic. The principal source of systemic IGF-1 is the liver, although most other tissues secrete and contribute to systemic IGF-1. Liver IGF-1 is considered to be the principal regulator of tissue growth. In particular, the IGF-1 secreted by the liver is believed to synchronize growth throughout the body, resulting in a homeostatic balance of tissue size and mass. IGF-1 secreted by peripheral tissues is generally considered to be autocrine or paracrine in its biological action.

Systemic IGF-1 also has hypothalamic and pituitary regulatory targets. The negative feedback loops cause down-regulation of GH secretion directly at the pituitary. The longer positive feedback loop, involving IGF-1 regulation at the hypothalamus, stimulates the secretion of GIH, which in turn inhibits the secretion of growth hormone by the pituitary. The latter is a relatively unusual negative feed-forward regulatory process. In addition, a shorter negative feedback loop is shown that involves direct IGF-1 action on the pituitary, leading to down-regulation of GH secretion. Similar feedback loops exist for all the major endocrine hormones, and many subtle nuances modulate each regulatory loop.

The Hypothalamic-Pituitary Axis

The hypothalamus is located below the thalamus and just above the brain stem and is composed of several domains (nuclei) that perform a variety of functions. The hypothalamus forms the ventral portion of the region of the brain called the diencephalon. Anatomically the hypothalamus is divided into three broad domains termed the posterior, tuberal, and anterior regions. Each of these three regions is further subdivided into medial and lateral areas. The various nuclei of the hypothalamus constitute the functional domains of the various hypothalamic areas. A few of the specific nuclei of the hypothalamus include the paraventricular nucleus (PVN) which is located in the anterior medial area and is involved in oxytocin and vasopressin release from the pituitary and the arcuate nucleus of the hypothalamus (ARC, also abbreviated ARH), the dorsomedial hypothalamic nucleus (DMH), and the ventromedial nucleus (VMN) all of which are located in the tuberal medial area. The ARC is involved in control of feeding behavior as well as secretion of various pituitary releasing hormones, the DMH is involved in stimulating gastrointestinal activity, and the VMN is involved in satiety (sensation of being full). The most important overall function of the hypothalamus is to link the central nervous system to the endocrine system via the pituitary gland (also termed the hypophysis). The hypothalamus is involved in the control of certain metabolic processes as well as other functions of the autonomic nervous system. With respect to this discussion the hypothalamus synthesizes and secretes a variety of neurohormones, referred to as hypothalamic-releasing factors, that act upon the pituitary to direct the release of the various pituitary hormones.

Diagrammatic representation of the interactions between the hypothalamus and the pituitary. The hypothalamic releasing and inhibiting hormones exert their effects on the release of anterior pituitary hormones. Oxytocin and vasopressin (antidiuretic hormone) are released directly from hypothalamic axons that terminate in the posterior pituitary, and the hormones are secreted from there directly into the systemic circulation.

The pituitary gland has two lobes called the posterior and anterior lobes. Each lobe secretes peptide hormones that exert numerous effects on the body. Each of the pituitary hormones is described in detail in the following sections. It is the aim of this discussion to provide the background for understanding what pituitary hormones are released and what are the triggers for their release. The posterior pituitary excretes the two hormones, oxytocin and vasopressin. The anterior pituitary secretes six hormones: adrenocortiocotropic hormone (ACTH, also called corticotropin), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH), and prolactin (PRL). The hormone ACTH is derived from a large precursor protein identified as pro-opiomelanocortin (POMC) as describe below. The secretion of the anterior pituitary hormones is under control of the hypothalamus, hence the description of the system as the hypothalamic-pituitary axis. The secretion of the hormones ACTH, TSH, FSH , LH, and GH are stimulated by signals from the hypothalamus, whereas, PRL secretion is inhibited by hypothalamic signals.

The secretion of anterior pituitary hormones results in response to hypophysiotropic hormones that are carried in the portal hypophysial vessels from the hypothalamus to the pituitary. These hypothalamic hormones are commonly referred to as releasing or inhibiting hormones. There are six hypothalamic releasing and inhibiting hormones: corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), luteinizing hormone-releasing hormone (LHRH), growth hormone-releasing hormone (GRH), growth hormone release-inhibiting hormone (GIH, more commonly called somatostatin), and prolactin release-inhibiting hormone (PIH or PIF). Hypothalamic extracts also contain a prolactin-releasing substance (sometimes referred to as prolactin-releasing hormone, PRH). Several peptides found in the hypothalamus (e.g. TRH) can stimulate prolactin secretion so it is as yet unclear whether PRH is the physiologic prolactin-releasing substance. GnRH has been shown to stimulate the release of both FSH and LH and as a consequence the term GnRH is more appropriately used than LHRH.

The hypothalamic releasing and inhibiting hormones are secreted from the median eminence of the hypothalamus. The GnRH-secreting neurons are primarily in the medial preoptic area of the hypothalamus. The somatostatin-secreting neurons reside in the periventricular nuclei. The TRH-secreting and CRH-secreting neurons are found in the medial parts of the periventricular nuclei. The GRH-secreting neurons reside in the arcuate nuclei which is the same region that contains dopamine-secreting neurons. Most of the receptors for the hypophysiotropic hormones are GPCRs.

The Gonadotropins

The glycoprotein hormones are the most chemically complex family of the peptide hormones. All members of the family are highly glycosylated. Each of the glycoprotein hormones is an (α:β) heterodimer, with the α-subunit being identical in all members of the family. The biological activity of the hormone is determined by the β-subunit, which is not active in the absence of the α-subunit. The α-subunit gene (identified as chorionic gonadotropin, alpha: CGA) is located on chromosome 6q12–q21.

The molecular weight of the gonadotropins (follicle stimulating hormone, FSH; luteinizing hormone, LH, and human chorionic gonadotropin, hCG) is about 25,000 Daltons, whereas that of the thyroid tropic hormone, thyroid stimulating hormone (TSH) is about 30,000. Synthesis of FSH and LH occurs in the same cells of the anterior pituitary and secretion of both is controlled by the hypothalamic decapeptide hormone GnRH. All members of the glycoprotein family transduce their intracellular effects via their respective receptors and the associated G-protein, adenylate cyclase, second-messenger systems. The gonadotropins (LH, FSH and hCG) bind to cells in the ovaries and testes, stimulating the production of the steroid sex hormones estrogen, testosterone (T), and dihydrotestosterone (DHT). The synthesis of the sex hormones is reviewed in the steroid hormone page.

FSH and LH

The FSH β-chain gene (gene symbol = FSHB) is located on chromosome 11p13. The LH β-chain gene (gene symbol = LHB) is located on chromosome 19q13.32. The synthesis and release of FSH and LH is controlled by the action of the hypothalamic releasing factor GnRH. The function of GnRH is to induce an episodic release of both FSH and LH that determines the onset of puberty and ovulation in females. GnRH binds to its receptor on gonadotropes and initiates a signaling cascade that results in release of FSH and LH. The GnRH receptor gene (symbol = GNRHR) is located on chromosome 4q21.2 and encodes a member of the GPCR and Ca²⁺-dependent receptor family. The control of the hypothalmic-pituitary axis at the level of FSH and LH is controlled by several additional proteins including follistatin, activin, and leptin. Follistatin is a protein that binds to and inhibits proteins of the transforming growth factor-β family (TGFβ) of which activin is a member. Therefore, follistatin inhibits the activity of activin on promoting FSH synthesis and release.

In females FSH stimulates follicular development and estrogen synthesis by granulosa cells of the ovary. In males FSH promotes testicular growth and within the Sertoli cells of the testes it enhances the synthesis and secretion of T and DHT. In females, LH induces thecal cells of the ovary to synthesize estrogens and progesterone and promotes estradiol secretion. The surge in LH release that occurs in mid-menstrual cycle is the responsible signal for ovulation. Continuous LH secretion stimulates the corpus luteum to produce progesterone. In males, LH binds to Leydig cells of the testes to induce the secretion of T.

The FSH receptor (FSHR) is located on chromosome 2p21–p16 spanning 54kbp and contains 10 exons that encode a 678 amino acid glycosylated member of the GPCR family of receptors. Binding of FSH to its receptors results in activation of adenylate cyclase leading to increased PKA activity. The LH receptor is referred to as the LH-choriogonadotropin receptor (LHCGR). The gene for this receptor is also found on chromosome 2p21 and encodes a 674 amino acid glycosylated member of the GPCR family of receptors. The LHCGR contains a large extracellular domain that includes several leucine-rich repeats (LRR). There are other members of the GPCR family that contain LRR in their extracellular domains and this subfamily of receptors is referred to as the LRR-containing GPCR (LRG) family. The LHCGR is coupled a G-protein that activates adenylate cyclase resulting in increased PKA activity. The LHCGR is expressed in the ovary, thecal cells, stromal cells, luteinizing granulosa, and luteal cells of the ovary and in Leydig cells of the testes.

hCG

The β-chain gene of hCG (gene symbol = CGB) is located on chromosome 19q13.32. Human chorionic gonadotropin is produced only during pregnancy. The actions of hCG are exerted by binding of the hormone to the LHCGR in the luteal cells of the ovary. Initially the developing embryo synthesizes and secretes hCG. Following implantation the cells of the syncytiotrophoblast (part of the placenta) produce and secrete hCG. The production of hCG increases markedly after implantation; its appearance in the plasma and urine is one of the earliest signals of pregnancy and the basis of many pregnancy tests. The role of hCG during pregnancy is to prevent disintegration of the corpus luteum so as to maintain the synthesis of progesterone by this tissue.

Thyroid Stimulating Hormone (TSH)

As indicated above, TSH (also called thyrotropin) is a member of the glycoprotein hormone family and as such is composed of a common α-subunit encoded by the CGA gene and a unique β-chain. The β-chain of TSH is encoded by the TSHB gene (thyroid-stimulating hormone, β-chain) which is on chromosome 1p13 and contains 3 exons, the first of which is non-coding. Secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. TRH, a tripeptide, is synthesized by neurons in the supraoptic and supraventricular nuclei of the hypothalamus and stored in the median eminence. TRH is transported to the anterior pituitary via the pituitary portal circulation and binds to a specific receptor located on TSH- and prolactin-secreting cells. Binding of TRH to its receptor activates a typical PLCγ-mediated signaling cascade The TRH-induced signaling leads to TSH secretion as well as increased TSH transcription and post-translational glycosylation. The TRH-mediated release of TSH is pulsatile with peak secretion being exerted between midnight and 4am.

The synthesis and release of TSH is controlled by two pathways. The first is exerted by the level of T3 within thyrotropic cells which regulates TSH expression, translation and release. The second regulation is of course exerted by TRH as described above. While in the circulation TSH binds to receptors on the basal membrane of thyroid follicles. The receptors are coupled through G-protein activation of adenylate cyclase as well as PLCγ. The TSH receptor gene (symbol = TSHR) is on chromosome 14q31 and encodes a 764 amino acid glycosylated member of the GPCR family of receptors. The TSHR and the LHCGR proteins share a significant degree of homology. TSH binding to its receptor triggers a signaling cascade that results in increased thyrocyte cAMP, PKA, IP₃, and DAG leading to, in the short term, increased secretion of the thyroid hormones, thyroxin (T4) and triiodothyronine (T3). TSH-binding to its receptor also results in increased TSH synthesis and thyroid cell growth.

Chronic stimulation of the TSH receptor causes an increase in the synthesis of a major thyroid hormone precursor, thyroglobulin. Thyroglobulin produced on rough endoplasmic reticulum has a molecular weight of 660,000. It is glycosylated and contains more than 100 tyrosine residues, which become iodinated and are used to synthesize T3 and T4. Thyroglobulin is exoctosed through the apical membrane into the closed lumen of thyroid follicles, where it accumulates as the major protein of the thyroid and where maturation takes place. Briefly, a Na⁺/K⁺-ATPase-driven pump concentrates iodide (I^–) in thyroid cells, and the iodide is transported to the follicle lumen. There it is oxidized to I⁺ by a thyroperoxidase found only in thyroid tissue. The addition of I⁺ to tyrosine residues of thyroglobulin is catalyzed by the same enzyme, leading to the production of thyroglobulin containing monoiodotyrosyl (MIT) and diiodotyrosyl (DIT) residues. The thyronines, T3 and T4, are formed by combining MIT and DIT residues on thyroglobulin.

Mature, iodinated thyroglobulin is taken up in vesicles by thyrocytes and fuses with lysosomes. Lysosomal proteases degrade thyroglobulin releasing amino acids and T3 and T4, which are secreted into the circulation. These compounds are very hydrophobic and require a carrier protein for delivery to target tissues. In the plasma, T3 and T4 are bound to a carrier glycoprotein known as thyroxin-binding globulin and are disseminated throughout the body in this form. The feedback loop that regulates T3 and T4 production is a single short negative loop, with the T3 and T4 being responsible for down-regulating pituitary TSH secretion. Meanwhile, continuously secreted hypothalamic TRH is responsible for up-regulating TSH production. The TSH actually secreted by thyrotropes is the net result of the negative effects of T3 and T4 and the positive effect of TRH.

Thyroid hormones act by binding to cytosolic receptors very similar to steroid hormone receptors, and for this reason T3 and T4 are often classified along with the hydrophobic steroid hormones. The principal role of thyroid hormones is also like that of steroid hormones. In adults, the ligand receptor combination binds to thyroid hormone response elements in nuclear DNA and is responsible for up-regulating general protein synthesis and inducing a state of positive nitrogen balance.

Numerous congenital and acquired forms of hypothyroidism and hyperthyroidism are the result of alterations in the expression, processing, and function of the TSHR. The most common TSHR disorder resulting in hyperthyroidism (thyrotoxicosis) is Graves disease. Graves disease is caused by thyroid-stimulating autoantibodies (TSAb, also called thyroid-stimulating immunoglobulins, TSIs) which bind to and activate the human TSH receptor, leading to the thyrotoxicosis characteristic of this disease. TSAbs bind to the TSH receptor and mimic the TSH stimulation of the gland by increasing intracellular cAMP. The hyperactivated thyroid then secretes excessive T3 and T4. Graves disease is classified as a form of thyrotoxicosis, the name for the clinical syndrome resulting from tissues exposed to high levels of thyroid hormones. One theory proposed for the development of the TSAb is that there is a defect in suppressor T cells that allows helper T cells to stimulate B cells to produce thyroid autoantibodies. The clinical features of Graves disease are thyrotoxicosis, goiter (enlarged thyroid gland), an ophthalmopathy in the form of exophthalmos (eyes bulge out), and dermopathy in the form of pretibial myxedema (localized lesions of the skin, primarily in the lower legs, resulting from the deposition of hyaluronic acid).

At the other end of the spectrum are disorders that lead to hypothyroidism. Deficiency in iodine is the most common cause of hypothyroidism worldwide. Indeed the practice of producing iodized table salt was to stem the occurrence of hypothyroidism. When hypothyroidism is evident in conjunction with sufficient iodine intake it is either autoimmune disease (Hashimoto thyroiditis) or the consequences of treatments for hyperthyroidism that are the cause. In the embryo, thyroid hormone is necessary for normal development and hypothyroidism in the embryo is responsible for cretinism, which is characterized by multiple congenital defects and mental retardation. Because the neurological consequences of congenital hypothyroidism are severe neonatal screening for thyroid hormone levels at birth is routine. Most infants born with congenital hypothyroidism appear normal at birth. However, if left untreated the symptoms will include a thick protruding tongue, poor feeding, prolonged jaundice (which exacerbates the neurological impairment), hypotonia (recognized as "floppy baby syndrome"), episodes of choking, and delayed bone maturation resulting in short stature.

The Pro-Opiomelanocortin (POMC) Family

The POMC gene is located on chromosome 2p23.3. POMC is expressed in both the anterior and intermediate lobes of the pituitary gland. The primary protein product of the POMC gene is a 285 amino acid precursor that can undergo differential processing to yield at least 8 peptides, dependent upon the location of synthesis and the stimulus leading to their production. POMC is produced in the pituitary, the ARC of the hypothalamus, the nucleus of the solitary tract (NTS for latin term nucleus tractus solitarii; specialized cells within the medulla responsible for sensations of taste and visceral sensations of stretch), as well as in several peripheral tissues such as the skin and reproductive organs. Within the brain neurons that respond to POMC-derived peptides (termed POMCm neurons) are critical in the regulation of overall energy balance via the melanocortin peptides (primarily α-MSH; this is N-terminally acetylated MSH).

The processing of POMC involves glycosylations, acetylations, and extensive proteolytic cleavage at sites shown to contain regions of basic protein sequences. The proteases that recognize these cleavage sites are tissue-specific; thus, the physiologically active product of the anterior pituitary is ACTH (discussed in detail in the section below). Aside from ACTH, the activities of the melanocortins (primarily α-MSH) produced in the anterior pituitary are the best understood peptides derived from the POMC mRNA. The activities of the melanocortin peptides is discussed in the section following ACTH.

Processing of the POMC precursor protein. Cleavage sites consist of the sequences, Arg-Lys, Lys-Arg or Lys-Lys. Enzymes responsible for processing of POMC peptides include prohormone convertase 1/3 (PC1/3), prohormone convertase 2 (PC2), carboxypeptidase E (CPE), peptidyl α-amidating monooxygenase (PAM), N-acetyltrasferase (N-AT), and prolylcarboxypeptidase (PRCP). Adrenocorticotropic hormone (ACTH) and β-lipotropin are products generated in the corticotrophic cells of the anterior pituitary under the control of corticotropin releasing hormone (CRH). Alpha-melanocyte stimulating hormone (α-MSH), corticotropin-like intermediate lobe peptide (CLIP), γ-lipotropin and β-endorphin are products generated in the intermediate lobe of the pituitary under the control of dopamine. α-, β- and γ-MSH are collectively referred to as melanotropin or intermedin. The blue shaded boxes represent the heptapeptide sequence that constitutes the MSH core.

Many of the other POMC products are synthesized in other neural tissues that contain proteases with appropriate specificity. In human embryos and in pregnant women, the intermediate lobe is active and leads to the production of endorphins and enkephalins. These same endorphin-producing pathways are active in other neural tissues, and since they bind to the opiate receptors in other parts of the brain they are assumed to represent natural opiate-like analgesic compounds.

Adrenocorticotropic Hormone, ACTH

ACTH is a 39 amino acid peptide that is derived by post-translational modification from the 241 amino acid preproprotein, POMC. ACTH is the main physiologically active product of the actions of the hypothalamic releasing hormone, CRH, on the anterior pituitary. Although CRH is the primary stimulus for ACTH release, other hormones also exert effects on ACTH release. CRH stimulates a pulsatile secretion of ACTH with peak levels seen before waking and declining as the day progresses. Negative feedback on ACTH secretion is exerted by cortisol at both the hypothalamic and anterior pituitary levels. Thus, the primary product of the systemic actions of ACTH regulates the further actions of this corticotropic hormone. Additional factors that influence ACTH secretion include physical, emotional, and chemical stresses. These stressors include pain, cold exposure, acute hypoglycemia, trauma, depression, and surgery. The stress-mediated increases in ACTH secretion are the result of the actions of vasopressin and CRH.

The biological role of ACTH is to stimulate the production of adrenal cortex steroids, principally the glucocorticoids cortisol and costicosterone. ACTH also stimulates the adrenal cortex to produce the mineralocorticoid, aldosterone as well as the androgen, androstenedione. ACTH exerts its effects on the adrenal cortex by binding to a specific receptor that is a member of the melanocortin receptor family. The ACTH receptors is identified as MC2R for melanocortin-2 receptor. The ACTH receptor is a G-protein coupled receptor and ACTH binding triggers activation of adenylate cyclase, elevation of cAMP, and increased PKA activity of adrenal cortex tissue. The main effect of these events is to increase the activity of CYP11A1 (also called P450-linked side chain-cleaving enzyme, P450ssc, 20,22-desmolase, or cholesterol desmolase), which converts cholesterol to pregnenolone during steroid hormone synthesis. Because of the distribution of enzymes in the various adrenal cortex subdivisions, the principal physiological effect of ACTH is production of the glucocorticoids.

Secondary adrenal insufficiency occurs in patients with deficiencies in pituitary ACTH production or secretion. Whereas, primary adrenal insufficiency (adrenal hypoplasia) is characteristic of Addison disease which was originally diagnosed as the result of lesions in the adrenal glands caused by tuberculosis. Secondary adrenal insufficiency is characterized by weakness, fatigue, nausea, vomiting, and anorexia. On the opposite side of the abnormal ACTH spectrum are the adrenal hyperplasias. These include the congenital adrenal hyperplasias (CAH) and Cushing syndrome. The CAH are a family of inherited disorders that result from loss-of-function mutations in one of several genes involved in adrenal steroid hormone synthesis. Endogenous causes of Cushing syndrome are pituitary corticotrope adenomas resulting in excess ACTH production and secretion. The characteristic features of Cushing syndrome are psychiatric disturbances (depression, mania, and psychoses), central obesity, hypertension, diabetes, moon-shaped face, thin fragile skin, easy bruising, and purple striae (stretch marks). In addition, Cushing syndrome patients manifest with gonadal dysfunction that is characteristic of hyperadrogenism with excess body and facial hair (hirsutism) and acne.

POMC-Derived Melanocortins and Feeding Behavior

The POMC-derived melanocortin peptides include α-MSH, β-MSH, γ-MSH, ACTH^1-24, and ACTH^1-13–NH₂ (desacetyl-α-MSH; indicated as des-α-MSH in above Figure). The POMC-derived melanocortins belong to a family of peptides referred to as the melanocortin system. This system includes the POMC-derived melanocotins which exhibit agonist activities, the antagonist peptide agouti-related peptide (AgRP), the melanocortin receptors (MCR), and the melanocortin receptor accessory proteins (MRAPs). The MCR family of receptors consists of five identified members termed MC1R through MC5R.

The melanocortin system has been shown to be critical in the regulation of food intake and energy expenditure via a number of different assay systems involving both humans and animals. The details of the role of melanocortins in appetite regulation are discussed in the Gut-Brain Interactions page.

Vasopressin and Oxytocin

The principal hormones of the posterior pituitary are the nonapeptides oxytocin and vasopressin (also called arginine vasopressin, AVP). The amino acid sequences of vasopressin and oxytocin differ by only two amino acids. Both of these hormones are synthesized as prohormones in neural cell bodies of the hypothalamus and mature as they pass down axons in association with carrier proteins termed neurophysins. The axons terminate in the posterior pituitary, and the hormones are secreted directly into the systemic circulation. The neurophysins themselves are derived from the oxytocin and vasopressin pre-pro-proteins. The oxytocin pre-pro-protein contains neurophysin I and the vasopressin pre-pro-protein contains neurophysin II. The human genes coding for pre-provasopressin-neurophysin II (prepro-AVP-NPII) and prepro-oxytocin-neurophysin I (prepro-OT-NPI) are similar in their intron-exon structure and they are linked together with 12kb of intervening DNA. Interestingly, the two genes are transcribed from opposite DNA strands.

Vasopressin is also known as antidiuretic hormone (ADH), because it is the main regulator of body fluid osmolarity. The designation arginine vasopressin is used when discussing vasopressins from different mammals as marsupials and pigs produce a vasopressin peptide where the arginine is replaced by a lysine and is thus, referred to as lysine vasopressin. The secretion of vasopressin is regulated in the hypothalamus by osmoreceptors which sense water and Na⁺ concentration and stimulate increased vasopressin secretion when plasma osmolarity increases. The secreted vasopressin increases the reabsorption rate of water in kidney tubule cells, causing the excretion of urine that is concentrated in Na⁺ and thus yielding a net drop in osmolarity of body fluids. Vasopressin deficiency leads to production of large volumes of watery urine and to polydipsia (increased desire for fluid intake). These symptoms are diagnostic of a condition known as diabetes insipidus. Diabetes insipidus has numerous causes that include effects on both the pituitary and kidneys.

Vasopressin binds plasma membrane receptors which activate signaling events through G-proteins coupled to the cAMP second messenger system or through the PLCγ pathway. There are three kinds of vasopressin receptors designated V_1A, V_1B, and V₂. The V_1A (gene symbol = AVPR1A) and V_1B (gene symbol = AVPR1B) receptors signal via hydrolysis of PIP₂ resulting in increased intracellular Ca²⁺ concentration. The V₂ receptors activate adenylate cyclase and result in increased cAMP levels. V₁ receptors are found in blood vessels and vasopressin binding triggers vascular contraction resulting in increased blood pressure. The V₂ receptors are found primarily in the collecting ducts of the kidneys and are responsible for triggering vasopressin-mediated water retention, thereby, affecting osmolarity. Mutations in the gene encoding the V₂ receptor (gene symbol = AVPR2) are responsible for X-linked nephrogenic diabetes insipidus.

Oxytocin is produced in the magnocellular neurosecretory cells of the hypothalamus and then stored in axon terminals of the anterior pituitary. While stored in the pituitary, oxytocin is bound to neurophysin I in Herring bodies. Secretion of oxytocin is stimulated by electrical activity of the oxytocin cells of the hypothalamus. The actions of oxytocin are elicited via the interaction of the hormone with high affinity receptors. The oxytocin receptor is a G-protein coupled receptor (GPCR) whose affinity for ligand is dependent upon Mg²⁺ and cholesterol, both of which act as positive allosteric regulators. Oxytocin secretion in nursing women is stimulated by direct neural feedback obtained by stimulation of the nipple during suckling. This response to oxytocin is referred to as the "let-down response". Its physiological effects include the contraction of mammary gland myoepithelial cells, which induces the ejection of milk from mammary glands. The other primary action of oxytocin is the stimulation of uterine smooth muscle contraction leading to childbirth. The uterine effect of oxytocin is due, in part, to increased production and release of the prostaglandin PGF_2α from the myometrium and to a lesser extent from the decidua.

In males the circulating levels of oxytocin increase at the time of ejaculation. It is believed that the increase in oxytocin levels causes increased contraction of the smooth muscle cells of the vas deferens thereby propelling the sperm toward the urethra.

Growth Hormone

Growth hormone (GH), human chorionic somatomammotropin, hCS (also called human placental lactogen, hPL), and prolactin (PRL) comprise the growth hormone family. All have about 200 amino acids, 2 disulfide bonds, and no glycosylation. Although each has special receptors and unique characteristics to their activity, they all possess growth-promoting and lactogenic activity. Mature GH (22,000 daltons) is synthesized in acidophilic pituitary somatotropes as a single polypeptide chain. Because of alternate RNA splicing, a small amount of a somewhat smaller molecular form is also secreted. Growth hormone and hCS are proteins encoded by genes in a cluster of five genes termed the human growth hormone/human chorionic somatomammotropin (GH/CS) gene cluster that spans 6.6kbp on chromosome 17q22–q24. In addition to the GH gene (symbol = GHN) and CS genes (gene symbol = CSA), this gene cluster contains the chorionic somatomammotropin hormone 2, CSH2 gene (gene symbol = CSB), the chorionic somatomammotropin-like1 , CSHL1 gene (gene symbol = CSL), and the growth hormone 2, GH2 gene (gene symbol = GHV which stands for growth hormone variant). Expression of GH occurs only in the pituitary whereas, the other four genes in the GH/CS gene cluster are expressed only in placental tissues.

While details of the method of signal transduction by the members of the GH family of tropic hormones remain unclear, PKC activity has been demonstrated to correlate directly with the biological effects of PRL and GH. This appears to indicate that the PKC signal transduction pathway is operative in transducing signals for the GH family of hormones.

The role of growth hormone in regulating IGF-1 production was noted above. Humans respond to natural or recombinant human or primate growth hormone with appropriate secretion of IGF-1, but growth hormone of other species has no normal biological effect in man. The latter is puzzling because interspecies GH homologies are quite high in many cases, and most other species respond well to human growth hormone. In humans, growth hormone promotes gluconeogenesis and is consequently hyperglycemic. It promotes amino acid uptake by cells, with the result that GH therapy puts an organism into positive nitrogen balance, similar to that seen in growing children. Finally, growth hormone is lipolytic, inducing the breakdown of tissue lipids and thus providing energy supplies that are used to support the stimulated protein synthesis induced by increased amino acid uptake.

There are a number of genetic deficiencies associated with GH. GH-deficient dwarfs lack the ability to synthesize or secrete GH, and these short-statured individuals respond well to GH therapy. Pygmies lack the IGF-1 response to GH but not its metabolic effects; thus in pygmies the deficiency is post-receptor in nature. Finally, Laron dwarfs have normal or excess plasma GH, but lack liver GH receptors and have low levels of circulating IGF-1. The defect in these individuals is clearly related to an inability to respond to GH by the production of IGF-1. The production of excessive amounts of GH before epiphyseal closure of the long bones leads to gigantism, and when GH becomes excessive after epiphyseal closure, acral bone growth leads to the characteristic features of acromegaly.

Human Chorionic Somatomammotropin (hCS)

Human chorionic somatomammotropin is produced by the placenta late in gestation. This hormone has also been called human placental lactogen (hPL) and chorionic growth hormone-prolactin (CGP). The hCS gene is found in the human growth hormone/human chorionic somatomammotropin (GH/CS) gene cluster as described above in the Growth Hormone section. The amino acid composition of hCS is similar to human growth hormone. Evidence suggests that due to the similarities between growth hormone, hCS and prolactin they likely evolved from a single progenitor hormone gene. At its height the hormone is secreted at a rate of about 1 g/day, the highest secretory rate of any known human hormone. However, little hCS reaches the fetal circulation. The amount of hCS that is secreted is proportional to the size of the placenta. Low levels of hCS during pregnancy are a sign of placental insufficiency. The biological actions of hCS are similar to those of growth hormone suggesting that it functions as a maternal growth hormone of pregnancy. The hormone induces the retention of potassium, calcium, and nitrogen, decreases glucose utilization and increases lipolysis.

Prolactin (PRL)

Prolactin is produced by acidophilic pituitary lactotropes. The PRL gene is located on chromosome 6p22.2–p21.3. Two promoter elements have been identified in the PRL gene that are located 5.5kbp apart. The 5'-promoter directs the expression of prolactin in decidualized endometrium as well as in lymphoblastoid cells. The more proximal promoter directs pituitary lactotrope expression and this promoter is controlled by the activity of the POU-domain transcription factor PIT1. Although the PRL gene is thought to have evolved from an ancestral hormone gene common to GH and hCS prolactin only shares 16% amino acid homology to these other two hormones. The prolactin protein is 198 amino acids in length with a molecular weight of 22,000 Daltons. Prolactin is known to bind zinc (Zn²⁺) and the binding of this metal stabilizes prolactin in the secretory pathway. Prolactin is the lone tropic hormone of the pituitary that is routinely under negative control by prolactin release-inhibiting hormone (PIH or PIF), which is now known to be dopamine. Decreased hypophyseal dopamine production, or damage to the hypophyseal stalk, leads to rapid up-regulation of PRL secretion. A number of other hypothalamic releasing hormones induce increased prolactin secretion; as a result, it is unclear whether a specific PRH exists for up-regulating PRL secretion. Prolactin does not appear to play a role in normal gonadal function yet hyperprolactinemia in humans results in hypogonadism.

Prolactin secretion increases during pregnancy and promotes breast development in preparation for the production of milk and lactation. Although prolactin serves this important function in breast development during pregnancy there is no evidence to indicate that it functions during normal breast tissue development before or during puberty. During pregnancy the increased production of estrogen enhances breast development but it also suppresses the effects of prolactin on lactation. Following parturition (birth) estrogen (as well as progesterone) levels fall allowing lactation to occur. This is to ensure that lactation is not induced until the baby is born.

The Pancreatic Polypeptide Family

The pancreatic polypeptide (PP) family of hormones comprises two gut hormones, pancreatic polypeptide (PP) and peptide tyrosine-tyrosine (PYY), as well as the central nervous system hormone neuropetide Y (NPY). Each of these peptide hormones contains 36 amino acids consisting of numerous tyrosines (hence the Y peptides nomenclature) and an α-amidation at the C-terminus. The three-dimensional structure of these hormones includes a hairpin-like motif referred to as the pancreatic polypeptide fold (PP-fold). The PP-fold is required for interaction of the hormones with specific G-protein coupled receptors (GPCRs).

Details of the actions of PP, PYY, and NPY are discussed in the Gut-Brain Interactions page.

Melanin-Concentrating Hormone, MCH

Melanin-concentrating hormone (MCH) was originally identified as a 19 amino acid cyclic peptide that induced the lightening of the skin in fish. Subsequently the peptide was identified in rodents to be overexpressed in response to fasting and also elevated in genetically obese mice (ob/ob mice).

Details of the actions of MCH are discussed in the Gut-Brain Interactions page.

The Orexins

The orexins constitute two neuroendocrine peptides derived from the same gene. These peptides are designated orexin A and orexin B. Details of the actions of the orexins are discussed in the Gut-Brain Interactions page.

Gastrointestinal Hormones and Peptides

There are more than 30 peptides currently identified as being expressed within the digestive tract, making the gut the largest endocrine organ in the body. The regulatory peptides synthesized by the gut include hormones, peptide neurotransmitters and growth factors. Indeed, several hormones and neurotransmitters first identified in the central nervous system and other endocrine organs have subsequently been found in endocrine cells and/or neurons of the gut. Visit the Table of Vertebrate Hormones page to see a more complete list of gastrointestinal peptides and hormones.

Adipose Tissue Hormones and Cytokines

Adipose tissue is not merely an organ designed to passively store excess carbon in the form of fatty acids esterified to glycerol (triacylglycerols). More complete information on the roles of adipose tissue in overall metabolism and inflammation can be found in the Adipose Tissue page.

Mature adipocytes synthesize and secrete numerous enzymes, growth factors, cytokines and hormones that are involved in overall energy homeostasis. Many of the factors that influence adipogenesis are also involved in diverse processes in the body including lipid homeostasis and modulation of inflammatory responses. In addition, a number of proteins secreted by adipocytes play important roles in these same processes. In fact recent evidence has demonstrated that many factors secreted from adipocytes are proinflammatory mediators and these proteins have been termed adipocytokines or adipokines. Members of this class of protein secreted from adipocytes include TNF-α, IL-6 and leptin. Listed in the Table below is only a subset of proteins known to be secreted by adipose tissue and the focus is on those that effect overall metabolic homeostasis and modulate inflammatory processes. As is clear from the Table, not all the proteins are unique to adipose tissue. Details of the structure and function of several proteins follows the Table.

Natriuretic Hormones

Natriuresis refers to enhanced urinary excretion of sodium. This can occur in certain disease states and through the action of diuretic drugs. At least 3 natriuretic hormones have been identified. Atrial natiuretic peptide (ANP) was the first cardiac natriuretic hormone identified. This hormone is secreted by cardiac muscle when sodium chloride intake is increased and when the volume of the extracellular fluid expands. Active ANP is a 28-amino acid peptide containing a 17-amino acid ring formed by intrachain disulfide bonding. Two smaller forms of ANP have also been isolated from the brain. A brain natriuretic peptide (BNP) (first isolated from porcine brain) has been identified and found in human heart and blood (but not human brain). BNP has different amino acids in its 17-amino acid ring and is encode for by a different gene. In humans, a third natriuretic peptide (CNP) is present in the brain but not in the heart.

The action of ANP is to cause natriuesis presumably by increasing glomerular filtration rate (its exact mechanism of action remains unclear). ANP induces relaxation of the mesangial cells of the glomeruli and thus may increase the surface area of these cells so that filtration is increased. Alternatively, ANP might act on tubule cells to increase sodium excretion. Other effects of ANP include reducing blood pressure, decreasing the responsiveness of adrenal glomerulosa cells to stimuli that result in aldostreone production and secretion, inhibit secretion of vasopressin and decreasing vascular smooth muscle cell responses to vasoconstrictive agents. These latter actions of ANP are counter to the effects of angiotensin II. In fact, ANP also lowers renin secretion by the kidneys thus lowering circulating angiotensin II levels.

Three different ANP receptors have been identified: ANPR-A, ANPR-B and ANPR-C. When ANP, BNP or CNP bind to receptor, an increase in guanylate cyclase activity results leading to production of cyclic GMP (cGMP). Both ANPR-A and ANPR-B proteins span the plasma membrane and their intracellular domains possess intrinsic guanylate cyclase activity. The exact function of the ANPR-C protein is unclear as this receptor does not contain an intracellular domain with intrinsic guanylylate cyclase activity. It is hypothesized that it may act through a G_q-protein that activates PLCγ and inhibits adenylate cyclase or that it acts simply as a clearance receptor removing natriuretic peptides from the blood.

Renin-Angiotensin System

The renin-angiotensin system is responsible for regulation of blood pressure. The intrarenal baroreceptor system is a key mechanism for regulating renin secretion. A drop in pressure results in the release of renin from the juxtaglomerular cells of the kidneys. Renin secretion is also regulated by the rate of Na⁺ and Cl^– transport across the macula densa. The higher the rate of transport of these ions the lower the rate of renin secretion. The only function for renin is to cleave a 10-amino acid peptide from the N-terminal end of angiotensinogen. This decapeptide is called angiotensin I. Angiotensin I is then cleaved by the action of angiotensin-converting enzyme (ACE) generating the bioactive hormone, angiotensin II. ACE removes 2 amino acids from the C-terminal end of angiotensin I.

Angiotensin II was also referred to as hypertensin and angiotonin. It is one of the most potent naturally occurring vasoconstrictors. The vasoconstrictive action of angiotensin II is primarily exerted on the arterioles and leads to a rise in both systolic and diastolic blood pressure. It is this action of angiotensin II on blood pressure that led to the development of a class of drugs called the ACE inhibitors for use as anti-hypertensive drugs. As the name implies, ACE inhibitors prevent ACE from converting angiotensin I to angiotensin II.

In individuals that are depleted of sodium or who have liver disease (e.g. cirrhosis), the pressive actions of angiotensin II are greatly reduced. These conditions lead to increased circulating levels of angiotensin II which in turn leads to a down-regulation in the numbers of angiotensin II receptors on smooth muscle cells. As a consequence, administration of exogenous angiotensin II to these individuals has little effect. Other physiological responses to angiotensin II include induction of adrenal cortex synthesis and secretion of aldosterone. Angiotensin II also acts on the brain leading to increased blood pressure, vasopressin and ACTH secretion and increased water intake. Angiotensin II affects the contractility of the mesangial cells of the kidney leading to decreased glomerular filtration rate. One additional effect of angiotensin II is to potentiate the release of norepinephrine.

Two distinct types of angiotensin II receptors have been identified, AT₁ and AT₂. The AT₁ receptors are classical serpentine G-protein coupled receptors (GPCRs). The AT₁ receptors are coupled to a G_q-protein that leads to activation of PLCγ. Although the AT₂ receptors are also serpentine, they do not appear to be coupled to activation of G-proteins.

Parathyroid Hormone (PTH)

Parathyroid hormone (PTH) is synthesized and secreted by chief cells of the parathyroid glands in response to systemic Ca²⁺ levels. There are four parathyroid glands that lie adjacent to the thyroid gland and consist of two superior glands and two inferior glands. The PTH gene is located on chromosome 11p15.3–p15.1 and consists of 3 exons encoding a 115 amino acid preproprotein. In the PTH mRNA exon 1 is untranslated, exon 2 encodes a 25-amino acid signal peptide and part of the prohormone, and exon 3 encodes the remaining part of the prohormone (6 amino acids) and the whole biologically active 84 amino acid (molecular weight of 9300 Daltons) PTH protein. As PTH is synthesized it is processed through the ER and Golgi apparatus where first the signal peptide is removed and then the propeptide with the active PTH molecule stored in dense neuroendocrine-type granules. There exists a related protein identified as PTH-related protein (PTHrP). PTHrP was identified originally as a protein causing severe hypercalcemia in patients with pheochromocytoma (PCC) which is a rare malignancy of the chromaffin cells of the adrenal medulla. The normal functions of PTHrP include roles in fetal bone development where it suppresses the maturation of chondrocytes so that the onset of hypertrophic differentiation during endochondral bone growth is delayed. In addition PTHrP exhibits antiproliferative effects in adults by regulating epidermal and hair follicle cell growth as well as inhibiting angiogenesis.

The Ca²⁺ receptor of the parathyroid gland responds to Ca²⁺ by increasing intracellular levels of PKC, Ca²⁺ and IP₃; this stage is followed, after a period of protein synthesis, by PTH secretion. The synthesis and secretion of PTH from chief cells is constitutive, but Ca²⁺ regulates the level of PTH in chief cells (and thus its secretion) by increasing the rate of PTH proteolysis when plasma Ca²⁺ levels rise and by decreasing the proteolysis of PTH when Ca²⁺ levels fall. The role of PTH is to regulate Ca²⁺ concentration in extracellular fluids. The feedback loop that regulates PTH secretion therefore involves the parathyroid, Ca²⁺, and the target tissues described below.

PTH acts by binding to cAMP– and PLCγ–activating plasma membrane receptors, initiating a cascade of reactions that culminates in the biological response. There are two receptors that recognize PTH identified as the PTH-1 and PTH-2 receptors (PTH1R and PTH2R). The PTH1R gene is located on chromosome 3p22–p21.1 spanning 32kbp and composed of 15 exons that encode a 585 amino acid protein. The PTH2R gene is located on chromosome 2q33. These receptors are coupled to G_s-proteins that activate adenylate cyclase resulting in increased cAMP and consequent activation of PKA, as well as G_q-proteins that activate PLCγ resulting in hydrolysis of membrane PIP₂ releasing IP₃ and DAG. The IP₃ stimulates release of intracellular Ca²⁺ stores and DAG activates PKC. The PTH-1 receptor is activated by both PTH and PTHrP, whereas, the PTH-2 receptor is activated only by PTH. Both receptors are related to a small sub-family of peptide hormone receptors that includes the receptors for ACTH, calcitonin, vasoactive intestinal peptide (VIP), and secretin. The PTH-1 receptor is found predominantly in bone and kidney. The body response to PTH is complex but is aimed in all tissues at increasing Ca²⁺ levels in extracellular fluids. PTH induces the dissolution of bone by stimulating osteoclast activity, which leads to elevated plasma Ca²⁺ and phosphate. In the kidney, PTH reduces renal Ca²⁺ clearance by stimulating its reabsorption; at the same time, PTH reduces the reabsorption of phosphate and thereby increases its clearance. Finally, PTH acts on the liver, kidney, and intestine to stimulate the production of the steroid hormone 1,25-dihydroxycholecalciferol (calcitriol), which is responsible for Ca²⁺ absorption in the intestine.

Although the primary function of PTH is to respond to reduced circulating Ca²⁺ levels and a portion of its action results in resorption of bone Ca²⁺, the use of recombinant PTH has proven beneficial in the treatment of osteoporosis. PTH increases bone turnover (resorption) but it also increases the formation of new bone and the latter effect on bone is more pronounced than resorption. Intermittent infusion of recombinant PTH (Forteo®) results in new bone formation and has shown efficacy in the treatment of the bone loss associated with osteoporosis. This PTH-induced phenomenon occurs as a result of the laying down of protein matrix and mineralization that occurs not only in the previous existing matrix, but in the new matrix that is formed. Patients receiving Forteo demonstrate an increase in bone density as measured at the osteoporosis center(s) as well as a decrease in fractures compared to other forms of treatment. Whereas, continuous elevation of PTH levels, as occurs in humans due to abnormal secretion from the parathyroid glands results in loss of bone mass leading to osteoporosis, intermittent elevation of PTH that occurs with the daily injections of Forteo has the opposite effect of building bone.

Calcitonin Family

Calcitonin is a 32-amino acid peptide secreted by C cells of the thyroid gland. Calcitonin is a hypocalcemic peptide that exerts its effects in numerous species by antagonizing the effects of PTH. In humans, however, the role of calcitonin in calcium homeostasis is of limited physiological significance. The circulating levels of calcitonin are low and extreme variations in these levels have not been associated with disruptions in calcium or phosphate homeostasis in humans. There are two calcitonin genes identified as α and β. The α gene (symbol = CALCA) is located on chromosome 11p15.2–p15.1 spanning 6.5kbp and composed of 6 exons. The β gene (symbol = CALCB) is located on the same chromosome location as the CALCA gene. Transcription of the CALCA gene yields two different mRNAs as a result of alternative splicing. The two mRNAs are translated into two distinctly different proteins with distinct biological activities. These two proteins are calcitonin and the neuropeptide calcitonin gene-related protein (CGRP, also identified as α-CGRP). Exon 1 of the CALCA gene is a non-coding exon and exons 2–4 encode calcitonin. The CGRP mRNA is composed of exons 1–3, 5 and 6. The CALCB gene encodes β-CGRP (also identified as CGRP-2) mRNA which is translated into CGRP in the central nervous system. The CGRP produced from the CALCB gene exhibits cardiovascular effects, neurotransmitter functions, and may serve a role in early development.

Calcitonin exerts its hypocalcemic effects primarily through inhibition of osteoclast-mediated bone resorption. Calcitonin has been shown to reduce the synthesis of osteopontin (OPN, also referred to as secreted phosphoprotein 1, SPP1), a protein made by osteoclasts and responsible for attaching osteoclasts to bone. Secondarily, calcitonin affects serum Ca²⁺ levels by the stimulation of renal Ca²⁺ clearance. These effects of calcitonin are the result of the interaction of the hormone with a specific receptor (identified as the CTR). The CTR is closely related to the PTH and the secretin receptors which together, as described above for PTH, form a distinct subfamily of GPCRs. The CTR gene (symbol = CALCR) is located on chromosome 7q21.3.

In humans the major benefits of calcitonin are its use in the treatment of osteoporosis and to suppress bone resorption in Paget disease. Paget disease (osteitis deformans) is a disorder of bone remodeling that results in accelerated rates of bone turnover and disruption of normal bone architecture. The naturally occurring calcitonins vary in amino acid sequence between species. The salmon calcitonin is 10–100 times more potent than mammalian species in lowering serum calcium levels and because of this activity it is used therapeutically such as in the treatment of Paget disease. The other medically significant fact related to calcitonin is its use as a biomarker for sporadic and inherited forms of medullary cancers.

In addition to calcitonin and CGRP, the calcitonin family of peptides includes amylin, adrenomedulin (AM) and adrenomedulin 2 (AM2, also known as intermedin). These peptides all interact with receptor complexes that contain the calcitonin receptor at their cores. The two G protein-coupled receptors (GPCRs) which are receptors for these peptides are the calcitonin receptor (CTR; gene symbol = CALCR) and the calcitonin receptor-like receptor (CLR, also known as CRLR). These belong to the sub-family of GPCRs known as the secretin-like or class B family of GPCRs (see the Signal Transduction page for more information on GPCR classes). CTR and CLR can form complexes with members of the membrane protein family called the receptor activity-modifying proteins (RAMPs), which consists of RAMP1, RAMP2 and RAMP3 in humans. RAMPs are type I transmembrane proteins composed of a large N-terminal extracellular domain, a single α-helical transmembrane domain and a short intracellular domain. These proteins regulate receptor pharmacology, receptor signaling, and receptor trafficking. RAMP association with CTR or with CLR generates multiple distinct receptor subtypes with different specificities for the calcitonin peptide family. CLR together with RAMP1 forms the CGRP receptor. In contrast, two AM receptors are formed by CLR and RAMP2 or RAMP3, respectively. Interaction of each of the three RAMPs with CTR form the amylin receptors described below.

Insulin, Glucagon and Somatostatin

The primary function of the pancreatic hormones is the regulation of whole-body energy metabolism, principally by regulating the concentration and activity of numerous enzymes involved in catabolism and anabolism of the major cell energy supplies.

The earliest of these hormones recognized was insulin, whose major function is to counter the concerted action of a number of hyperglycemia-generating hormones and to maintain low blood glucose levels. Because there are numerous hyperglycemic hormones, untreated disorders associated with insulin generally lead to severe hyperglycemia and shortened life span. Insulin is a member of a family of structurally and functionally similar molecules that include the insulin-like growth factors (IGF-1 and IGF-2), and relaxin. The tertiary structure of all 4 molecules is similar, and all have growth-promoting activities, but the dominant role of insulin is metabolic while the dominant roles of the IGFs and relaxin are in the regulation of cell growth and differentiation. For an extended discussion of the actions of insulin see the Insulin Action page.

Insulin is synthesized as a preprohormone in the β-cells of the islets of Langerhans. Its signal peptide is removed in the cisternae of the endoplasmic reticulum and it is packaged into secretory vesicles in the Golgi, folded to its native structure, and locked in this conformation by the formation of 2 disulfide bonds. Specific protease activity cleaves the center third of the molecule, which dissociates as C peptide, leaving the amino terminal B peptide disulfide bonded to the carboxy terminal A peptide.

Insulin secretion from β-cells is principally regulated by plasma glucose levels, but the precise mechanism by which the glucose signal is transduced remains unclear. One possibility is that the increased uptake of glucose by pancreatic β-cells leads to a concommitant increase in metabolism. The increase in metabolism leads to an elevation in the ATP/ADP ratio. This in turn leads to an inhibition of an ATP-sensitive K⁺ channel. The net result is a depolarization of the cell leading to Ca²⁺ influx and insulin secretion.

Chronic increases in numerous other hormones (including GH, hPL, estrogens, and progestins), up-regulate insulin secretion, probably by increasing the preproinsulin mRNA and enzymes involved in processing the increased preprohormone. In contrast, epinephrine diminishes insulin secretion by a cAMP-coupled regulatory path. In addition, epinephrine counters the effect of insulin in liver and peripheral tissue, where it binds to β-adrenergic receptors, induces adenylate cycles activity, increases cAMP, and activates PKA. The latter events induce glycogenolysis and gluconeogenesis, both of which are hyperglycemic and which thus counter insulin's effect on blood glucose levels.

Insulin secreted by the pancreas is directly infused via the portal vein to the liver, where it exerts profound metabolic effects. In most other tissues insulin increases the number of plasma membrane glucose transporters, but in liver glucose uptake is dramatically increased because of increased activity of the enzymes glucokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase (PK), the key regulatory enzymes of glycolysis. The latter effects are induced by insulin-dependent activation of phosphodiesterase, with decreased PKA activity and diminished phosphorylation of the regulatory glycolytic enzymes. In addition, phophatases specific for the phosphorylated forms of the glycolytic enzymes increase in activity under the influence of insulin. All these events lead to conversion of the glycolytic enzymes to their active forms and consequently a significant increase in glycolysis. In addition, glucose-6-phosphatase activity is down-regulated. The net effect is an increase in the content of hepatocyte glucose and its phosphorylated derivatives, with diminished blood glucose.

In addition to the latter events, diminished cAMP and elevated phosphatase activity combine to convert glycogen phosphorylase to its inactive form and glycogen synthase to its active form, with the result that not only is glucose funneled to glycolytic products, but glycogen content is increased as well.

Insulin generates its intracellular effects by binding to a plasma membrane receptor, which is the same in all cells. The receptor is a disulfide-bonded glycoprotein. One function of insulin (aside from its role in signal transduction) is to increase glucose transport in extrahepatic tissue is by increasing the number of glucose transport molecules in the plasma membrane. Glucose transporters are in a continuous state of turnover. Increases in the plasma membrane content of transporters stem from an increase in the rate of recruitment of new transporters into the plasma membrane, deriving from a special pool of preformed transporters localized in the cytoplasm.

In addition to its role in regulating glucose metabolism, insulin stimulates lipogenesis, diminishes lipolysis, and increases amino acid transport into cells. Insulin also modulates transcription, altering the cell content of numerous mRNAs. It stimulates growth, DNA synthesis, and cell replication, effects that it holds in common with the IGFs and relaxin.

Glucagon is a 29-amino acid hormone synthesized by the α-cells of the islets of Langerhans as a very much larger proglucagon molecule. Like insulin, glucagon lacks a plasma carrier protein, and like insulin its circulating half life is also about 5 minutes. As a consequence of the latter trait, the principal effect of glucagon is on the liver, which is the first tissue perfused by blood containing pancreatic secretions. The role of glucagon is well established. It binds to plasma membrane receptors and is coupled through G-proteins to adenylate cyclase. The resultant increases in cAMP and PKA reverse all of the effects described above that insulin has on liver. The increases also lead to a marked elevation of circulating glucose, with the glucose being derived from liver gluconeogenesis and liver glycogenolysis.

Glucagon also binds to receptors in adipose tissue resulting in increased activation of hormone-sensitive lipase (HSL). The actions of HSL lead to increased release of fatty acids stored in the triglycerides in adipose tissue. The released fatty acids enter the circulation, are bound by albumin and transported to various tissues for oxidation. In the liver the oxidation of fatty acids is necessary to provide the energy needed for gluconeogenesis which is activated in liver in response to glucagon.

Somatostatin, secreted by δ-cells of the pancreas, is a 14 amino acid peptide, identical to somatostatin secreted by the hypothalamus. In neural tissue somatostatin inhibits GH secretion and thus has systemic effects. In the pancreas, somatostatin acts as a paracrine inhibitor of other pancreatic hormones and thus also has systemic effects. It has been speculated that somatostatin secretion responds principally to blood glucose levels, increasing as blood glucose levels rise and thus leading to down-regulation of glucagon secretion.

Amylin

Amylin is a 37 amino acid peptide that is secreted from β-cells of the pancreas simultaneously with insulin in response to nutrient intake. Amylin has also been called islet amyloid polypeptide (IAPP). Amylin was originaly identified as a major component of diabetes-associated islet amyloid deposits, hence its original name IAPP. The structurally active form of amylin exists with an intrachain disulfide bond and an amidated C-terminus. When assayed by immunohistochemical means approximately 60% of amylin peptide present in the plasma is glycosylated. The functional significance of the glycosylation is currently unknown and when assayed in vitro the glycosylated peptide is biologically inactive. The primary actions attributable to amylin secretion are reduction in the rate of gastric emptying, suppression of food intake, and suppression of post-meal glucagon secretion. Collectively these three actions compliment the plasma glucose concentration regulating actions of insulin. The anorexigenic actions of amylin are most likely mediated within the CNS via neurons in the area postrema as evidenced by the fact that peripheral administration of amylin to animals results in neuronal activation in this region of the brain.  The plasma half-life of amylin is quite short being less than 15 minutes. The clearance of amylin from the plasma occurs via the kidneys both through renal excretion and renal peptidases associated with the vascular supply. A stable analog of amylin called pramlintide (Symlin ®) is used as an adjunct to insulin treatment for type 1 and type 2 diabetes. Patients who use pramlintide show a modest degree of weight loss. Current trials are being undertaken to establish the efficacy of pramlintide in the treatment of obesity in patients without diabetes.

Amylin exerts its effect via interaction with GPCR complexes of the secretin-like receptor family (GPCR class B receptors). There are three distinct receptor complexes that bind amylin. These complexes all contain the calcitonin receptor (CTR) as a core protein and either one of three receptor activity-modifying proteins (RAMPs), RAMP1, RAMP2 or RAMP3. The specific amylin receptors result from the dimerization of various splice variants of the calcitonin receptor (CTRa or CTRb) with either RAMP1, RAMP2 and RAMP3. These receptors are commonly referred to as AMY₁, AMY₂ and AMY₃ with either an a or b in the subscript designating which CTR splice variant of the calcitonin receptor is in the complex. Amylin receptors expressed in the nucleus accumbens, the dorsal raphe and the area postrema in the hind brain. Studies in rats have demonstrated that AMY_2a and AMY_3a are the amylin receptor subtypes localized to the area postrema which indicates that the satiety inducing effects of amylin are the result of activation of these two receptor subtypes. Within the area postrema, the key second messenger system associated with the amylin receptors appears to be cGMP. The calcitonin receptor-like receptor (CRLR) and both RAMP1 and RAMP2 are expressed in the subfornical organ and are likely responsible for the involvement of amylin in drinking behaviors. RAMP1 and RAMP2 but not RAMP3 have been shown to be expressed in the rat nucleus accumbens suggesting that the amylin receptor in the nucleus accumbens is either AMY₁ or AMY₂. The precise role of these amylin receptors in the nucleus accumbens haven not been well-established but it has been proposed that they may link food intake behavior and motor activity to amylin function. Peripheral injection of amylin demonstrates that the peptide crosses the blood-brain barrier resulting in access to a number of brain regions such as the cerebellum, midbrain, frontal cortex, parietal cortex, and occipital cortex.

Last modified: March 10, 2012